This invention relates to dry well temperature test systems and to systems especially designed to be portable for easy use on site in testing and calibration of various temperature sensing devices, such as thermo-couples, thermal switches, thermistors, thermometers, temperature control systems, and temperature indicators. A series of prior art temperature calibration systems are shown in U.S. Pat. Nos. 3,699,800; 3,738,174; 3,939,687; 4,075,882; and 4,079,618.
The prior art devices include an electrically heated, dry temperature well for receiving the probe or other temperature sensing device which is to be tested or calibrated. The devices also include some means for setting the desired temperature, a control circuit for controlling the input to the heat source in the temperature well in order to obtain the desired temperature, and some means for indicating the temperature of the heat source at the well.
The earlier of the above-identified devices operated only at temperatures higher than ambient. The two most recent of the U.S. Pat. Nos. 4,075,882 and 4,079,618, disclose a system utilizing a heat source which can operate at temperatures below ambient as well as above ambient. The invention of the present application is designed for use with such a heat source that can operate both below and above ambient, with the heat source providing for cooling below ambient temperature as well as heating above ambient temperature.
The prior art systems have all been analog in nature. A temperature sensing resistor is mounted in the heat source and is connected in a bridge circuit to provide a voltage signal which varies with the temperature of the heat source. This voltage signal is then utilized directly in controlling the power and the polarity of the power to the heat source for controlling the temperature of the heat source. A variety of problems are encountered in the analog circuits, including stability, determination of actual temperature from the resistance of the temperature sensing resistor which is nonlinear with temperature, calibration of the system itself by comparison with a standard temperature measuring device, and the expertise required of the operator in aligning the system.